Google is your friend – first hit – also you might first have a look at what serialization is.
It marks a member variable not to be
serialized when it is persisted to
streams of bytes. When an object is
transferred through the network, the
object needs to be ‘serialized’.
Serialization converts the object
state to serial bytes. Those bytes are
sent over the network and the object
is recreated from those bytes. Member
variables marked by the java transient
keyword are not transferred, they are
lost intentionally.
Example from there, slightly modified (thanks @pgras):
public class Foo implements Serializable
{
private String saveMe;
private transient String dontSaveMe;
private transient String password;
//...
}